Simple. ONI has many secrets. Noble Six, wounded but alive, managed to find a usable transport amongst the wreckage and fled the dying planet to Earth. Arriving after The Covenant were already on the way, ONI reassigned him to an ODST squad due to his substantial injuries and the fact that most of the other Spartans were dead and he is assigned to Sgt. Buck's squad, where he is silent due to the horrific memories of Reach.